1. Field of the Invention
The present invention generally relates to devices for measuring the electromagnetic interference (EMI) from electrical and electronic devices. More specifically, the present invention relates to a device for predicting the EMI radiated from interconnect cables of electrical and electronic devices.
2. Description of the Related Art
Electrical and electronic devices typically generate electromagnetic emission sometimes called EMI. Significant contributors to the overall EMI of a particular device are caused by common-mode (CM) currents on the interconnect cables and cables within the device. CM currents at the input/output (I/O) cables are the result of CM voltage at the connectors of the device. Normally, in the functional design of a device only the differential mode currents are considered and these currents constitute the normal or functional signals generated by the device. However, CM current may sometimes be superimposed on the differential mode currents, thereby causing EMI. The EMI generated by these devices propagate as conducted or radiate missions and may degrade the performance of the nearby electronic devices. Various national and international standards such as FCC part 15 and EN55022 regulate the conducted and radiated emission to permissible levels.
Several conventional techniques exist for measurement of EMI and troubleshooting EMI problems. One such method is the radiated-emission measurement technique. A device under test (DUT) is placed in an open area test site (OATS) on a turntable and the EMI is measured with an antenna placed usually at 10 meters from the device. FIG. 1 illustrates a CM model of a DUT with I/O cables showing the radiated-field measurement technique for determining the radiation characteristics of the DUT. A CM current I.sub.CM flows on an I/O cable causing radiation. The amplitude of the I.sub.CM depends on the CM voltage, the output impedance, and the impedance of the I/O cable. By rotating the DUT on the turntable, adjusting the height of the antenna, and finding the worst case I/O cable arrangement, the maximum radiation which varies in frequency can be found. The maximum measured field strength is therefore dependent on the position of the turntable, the antenna, and the cables. This procedure, however, is time consuming and typically takes a couple of hours to a day of testing.
In the case of shielded devices or small electrical devices with long interconnect cables attached, current clamps or a version called "absorption clamp", which directly measures the CM current on the cables are sometimes used for determining the EMI. By finding the amplitude of the CM current, the EMI may be determined. The current clamp method reduces time and provides a simpler "bench method." One disadvantage in using such a method, however, is that it is necessary to "maximize" readings by moving the clamp over at least 5-6 meters of the cable, and record the maximum at each frequency. Additionally, sometimes it is not feasible or practicable to measure a device with more than a couple of cables due to sizes of the cables involved with the current clamp. Further still, the ambient electromagnetic fields from nearby devices may also be present on the cables and the CM currents produced by such devices may also be measured with the current clamp, masking the CM noise under investigation.
Therefore, it is desirable to provide a device for measuring EMI generated from electrical and electronic devices that avoids the relatively complicated and time consuming procedures associated with the conventional methods of measuring EMI.